1. Field of the Invention
The present invention relates to a nitride semiconductor device and a manufacturing method thereof, and particularly to a nitride semiconductor device having an n-electrode provided over the back surface of a nitride semiconductor substrate and a manufacturing method thereof.
2. Description of the Background Art
Methods of fabricating electrodes on nitride semiconductor substrates made of, e.g. GaN, are disclosed in Japanese Patent Publication No. 2967743 and Japanese Patent Application Laid-Open Nos. 2003-347660, 2004-6718 and 2005-268769 (hereinafter referred to respectively as Patent Documents 1 to 4), for example, in which the substrate is polished, further subjected to dry-etching for removal of polishing defects, or further subjected to oxygen plasma treatment. In the manufacturing method disclosed in Patent Document 4, a doping process is intentionally applied during the dry-etching.
However, such conventional manufacturing methods are incapable of offering both of satisfactory ohmic properties and satisfactory adhesion. That is, in such methods, low-resistance ohmic properties cannot be obtained without application of high-temperature thermal treatment after the deposition of electrodes, or low-resistance ohmic properties can be obtained without thermal treatment but the resistance is undesirably increased by thermal treatment that is required for the device assembly process, or low-resistance ohmic properties can be obtained but sufficient adhesion cannot be obtained.
That is, the application of high-temperature thermal treatment may deteriorate electric characteristics of the device structure previously formed on the front side. Also, the variations of electric characteristics caused by thermal treatment during device assembly process may be avoided by assembling at low temperatures, but the characteristics might be varied with time after the assembly because of, e.g., the heat generated during operation. Also, improved adhesion may be obtained by designing specific electrode structures or by applying acid treatment to the surface, but it will impose restrictions on the processes or deteriorate the low resistance achieved by some means.